Children with ADHD are known to have extensive sleep disturbances and age-inappropriate increases in daytime and nighttime activity levels. Even when children with ADHD have been successfully treated with stimulants, the majority still has significant sleep difficulties. Deficits in activity levels in ADHD have been hypothesized to be associated with a dysfunction of endogenous arousal mechanisms. Despite the fact that the sleep/wake circadian rhythm is known to be involved in the control of arousal levels during the day and night, no studies have researched the circadian rhythms of children with ADHD. This general objective of this study is to determine the pattern of sleep/wake circadian rhythms in children with ADHD. Specific aims are: (1) to examine the relationship between daytime activity levels, quality and quantity of nighttime sleep, and biological sleep/wake cycles in children with ADHD and (2) to examine the effect of stimulant treatment (ST) on sleep and circadian rhythms in this population. Objective data on daytime activity levels and nighttime sleep will be collected using continuous ambulatory activity monitoring. Children's ADHD symptomatology will be subjectively and objectively assessed from several sources and compared to continuous sleep and circadian rhythms data. In order to successfully represent the endogenous sleep/wake circadian rhythm in children with ADHD, each child will be monitored for 4 days at 2 separate time periods, i.e. pre and post ST. Understanding the relationships between sleep quality and quantity, circadian rhythms, daytime activity levels, ADHD symptoms, and ST could lead to newer treatment approaches based on improving sleep and rhythms.